Diesel engine exhaust systems are facing increasingly stringent emission regulations, both for particulate emissions and NOx emissions. Vehicles equipped with diesel engines typically have diesel particulate filters for removing particulate matter from the exhaust stream. These filters capture carbon and hydrocarbon particulate from the exhaust. In addition to particulate filters for removing particulate matter, exhaust systems can be equipped with structures for removing other undesirable emissions such as carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx). Catalytic converters are typically used to remove CO and HC. NOx can be removed by structures such as lean NOx catalysts (LNC), selective catalytic reduction (SCR) catalysts and lean NOx traps (LNT).
Lean NOx catalysts are catalysts capable of converting NOx to nitrogen in an oxygen rich environment with the assistance of low levels of hydrocarbons. For diesel engines, hydrocarbon emissions are generally too low to provide adequate NOx conversion, thus hydrocarbons are injected into the exhaust stream upstream of the lean NOx catalysts. SCR's are also capable of converting NOx to nitrogen. However, in contrast to using hydrocarbons for conversion, SCR's use reductants such as urea or ammonia that are injected into the exhaust stream upstream of the SCR's. NOx traps use a material such as barium oxide to absorb NOx during lean burn operating conditions. During fuel rich operations, the NOx is desorbed and converted to nitrogen by catalysts (e.g., precious metals) within the traps.
However, all of these NOx reductions methods rely on knowing the NOx level, usually by using a NOx sensor or engine-NOx-map for predication of engine-out NOx, so that the after treatment device can be more accurately controlled to its maximum potential performance. Unfortunately, both engine-NOx-maps and NOx sensors are either very expensive to develop or not entirely reliable for practical transient use.
What are needed are methods to predict NOx levels based on less expensive and widely available engine parameters.